[ASan][Darwin] XFAIL test; atos does not show line numbers for globals

atos currently doesn't show source line numbers for global variables, so
global-location.cpp is failing after we switched from ASan-specific
metadata to normal symbolication.

See:
https://reviews.llvm.org/D127552

Author: Julian Lettner

compiler-rt/test/asan/TestCases/global-location.cpp

@@ -7,6 +7,9 @@
 // COFF doesn't support debuginfo for globals. For the non-debuginfo tests, see global-location-nodebug.cpp.
 // XFAIL: windows-msvc
 
+// atos currently doesn't show source line numbers for global variables.
+// XFAIL: darwin
+
 // CHECK: AddressSanitizer: global-buffer-overflow
 
 #include <string.h>